One example of an electronic graphic apparatus or system is described in our British Patent No. 2 089 625 and corresponding U.S. Pat. No. 4,514,818. This system includes a touch tablet and stylus combination for generating position signals designating the position of the stylus relative to the touch tablet. The user or operator paints or draws (hereinafter referred to simply as "paints") by selecting a color and a so called brush distribution from a range of predefined color and distribution functions. Movement of the stylus relative to the touch tablet causes the generation of a series of position signals. The system also includes a pixel processing circuit and a frame storage means which can store an array of pixels representing a picture. On moving the stylus to designate different positions, a patch of pixels in the frame storage means referenced by each designated position is modified in response to the selected color and distribution. All the pixels in the frame storage means, representing the picture, can moreover be read repeatedly and applied to a monitor to display the picture as it is built up by the operator.
The frame storage means for a system such as referred to in the preceding paragraph requires three ports, one for reading back pixels from the frame storage means to the processing circuit, one for writing the modified pixels from the processing circuit into the frame storage means and the third for repeatedly reading the pixels from the frame storage means to the monitor for display. Serial access is sufficient for reading the pixels as a video signal to the monitor, but random access is required via the first and second ports.
These requirements make it difficult to transfer pixels to the monitor at high rates, such as would be needed for example to refresh a monitor displaying a picture of high definition, say HDTV standard.
One object of the present invention is to reduce this difficulty.